1. Field of the Invention
This invention relates to hydraulic control systems, and more particularly to such systems in which a plurality of hydraulic actuators are to be precisely positioned in dependence on the magnitude of a similar plurality of electrical control signals.
2. Description of the Prior Art
There are numerous such control systems, and this invention would represent a significant advantage in connection with many of them. One exemplary and very significant application of such controls is in aircraft systems where hydraulic controls are provided for adjusting mechanical variables in jet aircraft engines. The gas turbine engines which are used to power conventional jet aircraft have commonly used hydraulic actuators for control of air valves, fuel valves, engine variable geometry, and the like. As engine designers attempt to achieve more and more performance from the gas turbine, the number of hydraulic actuators has increased significantly, and may approach 17 in number. Even gas turbine engines used on older commercial aircraft typically have on the order of six hydraulic actuators. In many cases, the actuators control functions which are critical, such as fuel supply, and on such critical functions, if control is lost, so is the engine.
Heretofore, each hydraulic actuator was provided with a device to convert an electrical input signal into a mechanical actuator position. Most typically, that had been done with a torque motor connected to and driving a hydraulic servo valve; the servo valve, in turn, controlled the supply of hydraulic fluid to the actuator. The torque motor, being dedicated to the associated actuator, could be driven for as long as additional actuator movement was desired. However, both torque motors and servo valves are fairly expensive, and both are fairly weighty components, particularly for aircraft applications where weight savings on the order of pounds can translate into substantial operating cost savings over the life of the aircraft.
Applicants are aware of a concept having been proposed to reduce weight and cost in such systems, by using a single pilot valve multiplexed among a plurality of actuators. In substance, the pilot valve has a spool which is rotated for multiplexing and which is positioned vertically by the torque motor to establish control positions. The spool and valve would be modified to provide a plurality of outlet ports at different angular positions of the spool such that the vertical control position of the valve combined with a plurality of angular multiplex positions could be used to sequentially deliver hydraulic fluid to a plurality of actuators. A position sensor on the rotary multiplexer would be used to coordinate multiplexed electrical signals for the pilot valve with the time slots of the multiplexer.
It is applicant's belief that a system of that type could not be reduced to practice for any but the most rudimentary systems because of a number of limitations, the most prominent one being the substantially reduced flow rate to any given actuator for a servo valve of any reasonable size. The flow rate reduction is a result of two factors -- 1) reduced flow through a pilot valve which is configured as a multiplexer, and 2) the fact of multiplexing itself which has flow going to an actuator only during its time slot. For a three channel system, the flow rate per cycle as compared to a standard non-multiplexed pilot valve would be reduced by about a factor of about 18. Thus, while in principle the system might work in applications where speed of response and fineness of control are not important criteria, in a jet engine control, for example, the concept would not appear to be workable.
Multiplexing of hydraulic circuits is not broadly new. It can be used for example in sharing a single transducer among a number of hydraulic or pneumatic channels, such as illustrated in Moore et al. U.S. Pat. No. 3,645,141. The opportunity to share a control servo valve among multiple actuators is also suggested in the literature, but on a manually controlled rather than a simultaneous multiplexed real time basis, insofar as applicant is aware. In contrast, in a true hydraulic multiplexed signal, control is being maintained over all of the channels, while servicing those channels individually and separately, but with sufficient frequency to maintain the outputs as representative of the inputs in substantially real time.
With respect to the prior multiplexing concept, insofar as applicant is aware, it has relied on a rotary multiplexer for sequentially activating the ports in the system. While rotary multiplexing can, in principle, be built in a very reliable fashion, acceptance of the rotary mode of operation imposes a number of drawbacks. Most particularly is the fact that the sequence is fixed by virtue of the mechanical connection of the channels to the rotary multiplexer. Each channel must be serviced in its sequence whether or not the channel has a demand for motion of its associated actuator. Thus, in the event that one of the channels demands a large actuator movement whereas another channel in the system is completely quiescent, each of the channels must be serviced in their assigned sequence and for the duration of their assigned time slot, even if service for the former is inadequate and service for the latter is superfluous. In summary, it is not only impossible to alter the sequence of channels to be serviced during operation of a rotary system, but it is also not possible to alter the length of the time slot of one channel with respect to any of its neighbors. This inherent inflexibility is undesirable in certain circumstances.